Scientists at Georgia State Universitys Neuroscience Institute and Center for Behavioral Neuroscience have for the first time identified the most likely reason why analgesicdrug treatment is usually less potent in females than males.

“Opioid-based narcotics such as morphine are the most widely prescribed therapeutic agents for the alleviation of persistent pain. However, it is becoming increasingly clear that morphine is significantly less potent in women compared with men. Until now, the mechanism driving the phenomenon was unknown,” said Anne Murphy, who conducted the research with Dayna Loyd.

Scientists through animal studies have shown that the previously reported differences in morphine’s ability to block pain in male versus female rats are most likely due to sex differences in mu-opioid receptor expression in a region of the brain called the periaqueductal gray area (PAG).

Located in the midbrain area, the PAG plays a major role in the modulation of pain by housing a large population of mu-opioid receptor expressing neurons. Morphine and similar drugs bind to these mu-opioid receptors and ultimately tell the brain to stop responding to pain signals to the nerve cells resulting in the reduced sensation of pain.

Scientists say the discovery is a major step toward finding more effective treatments for females suffering from persistent pain.

Reacting to the study, Dr Madhuri Bihari, head of department of neurology at AIIMS said: “There is a difference in reaction of analgesic drugs on male and female bodies even though it is slight. Pain reduces slightly more in men than women after popping a painkiller.”

“It was believed that it’s because of the faster metabolic rate among women. This study is, therefore, significant. How much it will help clinically is yet to be understood,” Dr Bihari said.

Using a series of anatomical and behavioural tests, Murphy and Loyd were able to determine that male rats have a significantly higher level of mu-opioid receptors in the PAG region of the brain compared with females.

This higher level of receptors is what makes morphine more potent in males because less drug is required to activate enough receptors to reduce the experience of pain.

Interestingly, when they used a plant-derived toxin to remove the mu-opioid receptor from the PAG, morphine no longer worked, suggesting that this brain region is required for opiate-mediated pain relief.

Additional tests also found females reacted differently to morphine depending on the stage of their estrous cycle.

Analgesic drug market in India has swelled over the years. Painkiller drug Voveran had emerged as the top brand in the domestic pharmaceutical market with the largest sales for the first seven months of 2007